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RFC 2919 / MAILING LIST IDENTIFICATION

Allocate an identifier in an authorized namespace

02

Domain ownership delegates the right to allocate list identifiers. Unmanaged .localhost identifiers offer a separate, weaker option.

DIAGRAMRead RFC section 2 ↗
RFC 2919 · § 2EXAMPLE
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Allocate an identifier in an authorized namespaceFour allocation cases. An authorized example.org administrator chooses garden.example.org independently of the delivery hostname. A separate unauthorized operator is stopped before allocating in that namespace. Without domain authority, the unmanaged localhost alternative includes a required random portion; a fixed 32-hex-character illustration and MMYYYY 102026 follow SHOULD guidance. The final scene shows two unmanaged allocations can collide, so syntax and randomness do not prove uniqueness or authority. No DNS lookup or random-number generation is performed.Unmanaged namespace: .localhost32 hex characters · illustrated valueda39efc25c530ad145d41b86f7420c3b102026.localhostAppend .102026.localhost to the value abovesame ID possibleGlobal uniqueness is NOT guaranteed
04
Keep the weaker guarantee visible.

Two unmanaged allocations can collide. The appearance of a long random string reduces risk but does not make .localhost globally unique or authenticate its creator.

Source for this drawing: RFC 2919 · Read RFC section 2 ↗

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KEY POINT

A domain-shaped identifier is an allocated name, not a DNS lookup.

  • A list owner MUST NOT generate an identifier in a domain namespace without authority. Identifiers outside .localhost MUST be globally unique among mailing lists.
  • A list-id subdomain is suggested, not required. The identifier can be independent of the delivery host; no host record or lookup is required here.
  • For unmanaged .localhost IDs some portion MUST be random. A creation MMYYYY subdomain and a 32-hex-character encoding of 128 random bits are SHOULD guidance. .localhost IDs are not guaranteed globally unique.
DETAILS

The namespace and label use dot-atom-text syntax, not a claim that every identifier is a resolvable hostname. The 32-hex value below is a fixed fictional illustration, not newly generated randomness. Namespace authority, uniqueness and randomness cannot be proved by inspecting syntax alone.

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